The present invention relates to lamp sockets, and more particularly to a such a lamp socket which easy and inexpensive to manufacture and, which firmly holds down the bulb when the bulb is mounted.
Various decorative strings and Christmas tree light sets are well-known and intensively used in western countries as well as most Asian countries during Christmas holidays. When installed, decorative strings and Christmas tree light sets are controlled to flash or to give different colors of light. In recent days, the requirement for a safety operation on decorative lighting devices has become more and more critical. UL and CSA define strict specifications on these products. The lamp socket for a regular decorative string or Christmas tree light set is generally comprised of an internally threaded plastic socket body having a first contact metal plate and a second contact metal plate mounted on the inside. The socket body has a wire groove on the top end thereof for mounting the electric wire, and two slots through which the top end of either contact metal plate projects. The contact metal plates have each a beveled top edge projecting out of either slot into the wire groove. When the electric wire is placed in the wire groove, a plastic socket cap is fastened to the socket body to hold down the electric wire, causing the beveled top edges of the contact metal plates pierced the insulator of either conductor of the electric wire to make a respective electric contact. The first contact metal plate has a curved bottom end transversely disposed inside the socket body at the top and stopped between two ribs. When the base of the lamp bulb is threaded into the socket body, the tip and ring contacts of the lamp bulb are respectively connected to the conductors of the electric wire through the contact metal plates. This structure of lamp socket is functional however, it has drawbacks. The major drawback of this structure of lamp socket is that the socket body may expand when hot, causing a gap produced between the base of the lamp bulb and the threaded inside wall of the socket body. Therefore, an electric shock may occur when one touches the lamp bulb.
There is known a lamp socket designed to eliminate the aforesaid drawback by fastening a contact metal socket to the socket body of the lamp socket on the inside through a screw joint to hold the bulb. Therefore, the ring contact of the bulb can be electrically connected to the second contact metal plate and the tip contact of the bulb can be extended out of a hole on the contact metal socket and to make an electric contact with the first contact metal plate. The contact metal socket has a serrated outward flange at the bottom engaging the inside wall of the socket body to secure the connection between the socket body and the contact metal socket. This structure of lamp socket eliminates the aforesaid drawback however, its manufacturing cost is high.